Strip support for 1.8 and 1.9 from JRuby

With the aim to reduce unnecessary space, this PR strips support from the vendorized JRuby for 1.8 and 2.0 libs. It also makes sure that logstash is run under the 1.9 mode.

Fits the point number four of #2137 

 
at this point it might make sense to have a CONSTANT that lists all regexp that we want to ignore, maybe?

```ruby
DISCARD_PATTERNS = Regexp.union [
       /^samples/
        /@LongLink/
      /lib\/ruby\/1.8/
       /lib\/ruby\/2.0/
]
```